The invention relates to an apparatus and method of automatically testing laser devices. More particularly, the invention relates to an apparatus and method of automatically testing laser diode sub-assemblies.
Laser diode assemblies are constructed using a vertical surface laser die mounted onto a post. The vertical surface laser die generates a divergent light, similar to a flashlight and requires an external lense to lase. The die is bonded to the top of a metal post that is used to provide a secure base for the die as well as conduct electricity, but most importantly to conduct heat away from the die. The post and one contact of the die are wired together, and a wire or thin metal ribbon is extended from the other contact on the die. The extended wire or thin metal ribbon is called a tab and serves only as an electrical contact. Such a configuration is referred to as a chip-on-post, or COP. At this stage, the die has yet to be tested. When testing the COP, the objective is to simulate actual conditions by applying power to the COP and measuring the light output characteristics. Two characteristics are typically measured to determine whether or not the die is functioning correctly, the power and the light spectrum. The spectrum of the light output generally measures the number and shape of pulses, and the power of the light measures the light power output versus the power input.
To test the COP, a mechanical mount is used to hold the post and the tab in place. In typical test situations, up to 5 amps are pulled through the COP. If the heat is not sufficiently dissipated, the tab and/or the die will burn up. The COP is typically mechanically clamped in a conductive metal fixture to draw away the heat generated during testing and act as an electrode.
Once the COP is mechanically and electrically clamped, the divergent light from the die needs to be focused and reflected using an external lens. That is, the light is focused into a parallel beam that makes a laser light. Lasing the divergent light is accomplished by properly positioning a lens within the path of the divergent light. This arrangement is referred to as a laser diode sub-assembly. FIG. 1 illustrates a laser diode sub-assembly of the prior art. A die 20 is bonded and wired to a post 10. A tab 30 is coupled to the die 20 and the post 10 via the wired connection. The post 10, the die 20 and the tab 30 together form a COP. When power is applied to the COP, a divergent light output 40 is generated. A lens 50 is properly positioned in the path of the divergent light output 40 to focus the divergent light 40 into laser light 60.
Conventionally, both the lens and the COP are mounted on optical mounts. The mounts holding the lens and COP are coupled to micrometers that move the mounts, and therefore the lens and COP, in very small, incremental steps. A human operator, usually a highly trained optical technician, manually turns the knobs on the micrometers to obtain proper COP and lens alignment. The operator turns the micrometer knobs that in turn move the lens and COP back and forth, up and down, as well as tilting to obtain proper alignment for lasing. The operator monitors the output of a measuring device while moving the knobs to determine if the lens is being moved in the proper direction. The operator continues to adjust the micrometers and monitor the measurement device until the lens and COP are approximately positioned. The operator continues to adjust the micrometers until the highest readings are obtained and then manually records the final measurements. Such a process is very time consuming and subject to operator expertise and training, not to mention the inability to securely couple data taken with the device tested. This process is also potentially dangerous since lasing of the divergent light is performed in an open, unsafe environment. Operators are required to wear laser-proof glasses to prevent injury, but accidents can happen.
The power and the spectrum of the laser light are tested. This requires two different tests, each test requiring a different detection device. One detection device is used to detect the spectrum or frequency of the laser light. This detection device is coupled to a spectrum analyzer to measure spectrum characteristics. A second detection device is used to detect the power of the laser output. This detection device is then coupled to a power meter and the results of the voltage-ampere curve and peak power output of the laser light are recorded. Once the first test is completed, the COP is then aligned the second detection device in order to perform the second test. Either the detectors are interchanged or the COP may be moved to a different test setup. As a rule, the lens will need to be re-aligned when performing the second test. Even when the lens does not need to be re-aligned, changing the detection devices or moving the COP into different setups is very time consuming and prone to errors.
When performing a test on a different COP, re-alignment of the lens is necessary due to the irregularities from one COP to the next. Each die will be different due to the nature of the wafer fabrication process. Additionally, there are irregularities associated with mounting the die to the post. Mounting of the die is done using a special epoxy that mechanically and thermally bonds the die to the post. The assembly is then put in an oven to allow curing of the epoxy before the next step. Manufacturing processes associated with bonding a die on a post specify tolerances within which the die is to be placed on the post. Preferably, die are placed in the center of the post and flat relative to the top of the post. However, manufacturing processes introduce undesirable variables, so that every die is not positioned exactly the same as the next die.
A preferred embodiment of the present invention automatically tests a laser diode sub-assembly using a testing system. The testing system automatically loads, positions, transports, aligns, tests, records data and unloads each serialized laser diode sub-assembly to be tested without need of user interaction. While the preferred embodiment of the present invention is used to perform automated testing of COP laser diode subassemblies, the testing system of the present invention is also capable of performing automated testing on most laser diode assemblies as well as other devices under test (DUTs) with power up to 25 watts.
According to an aspect of the present invention, a testing system includes a handling system for automatically loading and positioning within a given tolerance each of a plurality of laser diode sub-assemblies, an optical system for automatically receiving each laser diode subassembly from the handling system and automatically performing one or more tests to measure functionality of each laser diode sub-assembly, a detection system for detecting characteristics associated with one or more tests performed by the optical system for each laser diode sub-assembly, and a control system for automatically receiving detected characteristics from the detection system, comparing the detected characteristics to stored expected characteristics for a properly functioning laser diode sub-assembly thereby forming a comparison, and providing control instructions to the optical system based on the comparison. The one or more tests are performed by automatically positioning a lens such that light generated from the laser diode sub-assembly is formed into laser light, and each test measures a desired characteristic of the laser light. The detection system includes one or more detection devices, each detection device corresponding to one of the one or more tests performed. The testing system also includes a mirror assembly properly positioned to direct the laser light to the one detection device corresponding to the test currently being performed by the optical system. Each detection device detects data associated with the desired characteristic and transmits the detected data to the control system. The control system periodically sends control instructions to the optical system in response to the received detected data to incrementally adjust the position of the lens until an optimal lens position is obtained. The optical system also includes one or more motors used to incrementally adjust the position of the lens. The handling system positions the laser diode sub-assembly at a specific location within a collet and maneuvers the collet with the positioned laser diode sub-assembly to a specific location within the optical system. The control system stores the detected characteristics and associates the detected characteristics to the laser diode sub-assembly from which the detected characteristics are generated. The handling system comprises a loading device for automatically loading and unloading each of the laser diode sub-assemblies into and out of one of one or more collets, a carousel including one or more nests, each nest for supporting one of the one or more collets, wherein the carousel moves each collet from a loading position, to one or more testing positions, and to an unloading position. Each collet supported by the carousel is thermally conditioned by a thermo-electronic controller to within a first thermal tolerance. The optical system automatically secures the collet into a specified position by actuating a thermal conditioning apparatus against a bottom surface of the collet. The collet, once moved to one of one or more testing positions, is automatically removed from the carousel and loaded into the optical system by the handling system. The testing system also includes a thermal conditioning apparatus within the optical system for thermally conditioning the collet to within a second thermal tolerance and to dynamically thermally condition the collet as power is applied to the laser diode sub-assembly within the collet during one of the one or more tests performed in order to maintain the thermal conditioning of the collet within the second thermal tolerance. Each laser diode sub-assembly to be loaded into one of the one or more collets is carried over a camera by the loading device prior to loading the laser diode sub-assembly into the collet wherein the camera scans an identification marking on the laser diode sub-assembly for tracking the laser diode sub-assembly within the testing system and to associate the detected characteristics of each test performed on the laser diode sub-assembly to the identification marking of the laser diode sub-assembly. Each laser diode sub-assembly to be unloaded from handling system is carried over a camera by the loading device subsequent to removing the laser diode sub-assembly from the collet wherein the camera scans the identification marking on the laser diode sub-assembly for identifying the laser diode sub-assembly and confirming that the laser diode sub-assembly is unloaded from the handling system. The carousel incrementally rotates through a plurality of positions, wherein the loading position and the one or more testing positions are included within the plurality of positions, further wherein the handling system uses sensors to monitor that the carousel accurately increments from one position to the next. The sensors also monitor that the laser diode sub-assembly is positioned within the collet within the given tolerance.
According to another aspect of the present invention, an automatically aligning optical testing system includes means for securing a laser diode sub-assembly into a fixed position, a lens for lasing a light output from the laser diode sub-assembly, a vertical means for clamping the lens into a fixed vertical position, a rotational means for clamping the lens into a fixed rotational position, means for applying power to the laser diode sub-assembly, a mirror assembly for directing the laser light to a detection device, a first motor coupled to the vertical means for clamping for moving the vertical means for clamping in a vertical direction, thereby vertically moving the lens, a second motor coupled to the rotational means for clamping for rotating the rotational means for clamping, thereby rotating the lens, and a receiving circuit coupled to the first motor and to the second motor for receiving control instructions from a control system, wherein the control system determines the control instructions by analyzing the laser light detected by the detection device, wherein the control instructions instruct the first and second motors to incrementally step thereby adjusting the position of the lens relative the fixed position of the laser diode sub-assembly, further wherein the receiving circuit iteratively receives control instructions to adjust the position of the lens until an optimal lens position is reached at which time testing of the laser diode sub-assembly is performed. The first and the second motors are picomotors. The means for securing the laser diode sub-assembly into the fixed position includes positioning the laser diode sub-assembly into a collet and actuating a thermal conditioning apparatus against a bottom surface of the collet. The thermal conditioning apparatus thermally conditions the collet to within a thermal tolerance and dynamically thermally conditions the collet as power is applied to the laser diode sub-assembly in order to maintain the thermal conditioning of the collet within the thermal tolerance. The automatically aligning optical testing system also includes an air cylinder coupled to the mirror assembly for rotating a mirror within the mirror assembly thereby directing the laser light to one of one or more detection devices.
According to yet another aspect of the present invention, a method of automatically performing optical tests on a device includes automatically loading a laser diode sub-assembly within a holding device, automatically positioning the holding device within a lens assembly, automatically detecting desired characteristics of a laser light output from the laser diode sub-assembly, automatically comparing the detected characteristics to stored expected characteristics for a properly functioning laser diode sub-assembly, automatically providing control instructions to adjust the position of the lens assembly relative to the laser diode sub-assembly based on the comparison, automatically repeating automatically detecting, automatically comparing and automatically providing until the lens assembly is adjusted into an optimal position, and automatically determining test results by analyzing the detected desired characteristics while the lens assembly is in the optimal position. The method further includes automatically re-directing the laser light to detect different desired characteristics of the laser light. The method further includes automatically comparing the different detected characteristics to stored expected characteristics for a properly functioning laser diode sub-assembly, automatically providing control instructions to adjust the position of the lens assembly relative the laser diode sub-assembly based on the comparison, repeating these steps until the lens assembly is adjusted into an optimal position, and determining test results by analyzing the different detected desired characteristics while the lens assembly is in the optimal position.
According to still yet another aspect of the present invention, an apparatus for securing, locating, electrically and thermally contacting a device under test comprises a block of thermally conductive material, a channel extending vertically through the block and extending horizontally from a front surface of the block towards a back surface of the block without reaching the back surface and a positioning hole extending vertically through the block and intersecting the channel, wherein the positioning hole includes a top portion with a width larger than a width of a bottom portion of the positioning hole, further wherein the top portion is sufficiently long as to position the device under test within the positioning hole such that a bottom surface of the device under test rests at a top of the bottom portion of the positioning hole and a top surface of the device under test rests above a top surface of the block, wherein the channel is widened from a standard configuration to an extended configuration by applying a prying means to the channel, and the channel returns to the standard configuration from the extended configuration upon removing the prying means from the channel. The block is preferably made of beryllium copper. The channel includes a wedge-shaped opening at the front surface of the block. A cross-section of the top portion of the positioning hole matches a cross-section of the device under test. The prying means is a wedge and the channel is widened to the extended configuration by pressing the wedge into the wedge-shaped opening of the channel. The width of the top portion of the positioning hole is larger than a width of the channel and equal to a width of the device under test. The cross-section of the top portion of the positioning hole and the device under test is circular.
According to yet another aspect of the present invention, an apparatus for securing a device under test includes a beryllium copper block, a channel horizontally bisecting the block such that the channel extends vertically from a top surface of the block through a bottom surface of the block, and the channel extends horizontally from a front surface of the block past a center of the block but not as far as a back surface of the block, wherein the channel includes a wedge-shaped opening at the front surface of the block, and a positioning hole extending from a top center of the top surface of the block to a bottom center of the bottom surface of the block and intersecting the channel, wherein a cross-section of the positioning hole matches a cross-section of the device under test, further wherein the positioning hole includes a top portion with a width larger than a width of the channel and equal to a width of the device under test, and a bottom portion with a width less than the width of the device under test, further wherein the top portion is sufficiently long as to position the device under test within the positioning hole such that a bottom surface of the device under test rests at a top of the bottom portion of the positioning hole and a top surface of the device under test rests above the top surface of the block, wherein the width of the channel is widened from a standard width to an extended width by pressing a wedge into the wedge-shaped opening of the channel and the width of the channel returns to the standard width when the wedge is removed from the wedge-shaped opening. The device under test is a laser diode sub-assembly. The cross-section of the positioning hole and the laser diode sub-assembly is circular.